Wash. Rev. Code § 35.43.100
The council may continue the hearing upon any petition or resolution provided for in this chapter and shall retain jurisdiction thereof until it is finally disposed of. The action and decision of the council as to all matters passed upon by it in relation to any petition or resolution shall be final and conclusive. No lawsuit whatsoever may be maintained challenging the jurisdiction or authority of the council to proceed with the improvement and creating the local improvement district or in any way challenging the validity thereof or any proceedings relating thereto unless that lawsuit is served and filed no later than thirty days after the date of passage of the ordinance ordering the improvement and creating the district or, when applicable, no later than thirty days after the expiration of the thirty-day protest period provided in RCW 35.43.180.
RCW 35.43.100